The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to the plurality of torque transmitting devices within the transmission. The pressurized hydraulic fluid delivered to the torque transmitting devices is used to engage or disengage the devices in order to obtain different gear ratios.
In energy efficient motor vehicles, the engine may be stopped and started during appropriate conditions in order to improve the fuel economy of the vehicle. During an engine stop, the pump is no longer active and providing pressurized hydraulic fluid to the valve body. During engine restart, the pump may be delayed in providing pressurized hydraulic fluid to the valve body due to the time needed to pull hydraulic fluid from a sump to the pump. This delay may be increased in cold conditions. Accordingly, there is room in the art for improved hydraulic control circuits that reduce the amount of complex components while improving efficiency and controllability of the system during restart of the engine.